Electronic parts such as lead frames and connector pins are often subject to partial silver plating. Sometimes, silver coatings are deposited beyond the necessary regions to be plated. It is then required to strip and remove extra portions of the silver coating extending beyond the necessary regions. It is also a common practice to form patterned silver coatings by plating silver on a wider article area and then stripping unnecessary silver coating portions while retaining necessary silver coating portions.
Overcoating, that is, deposition of silver coatings outside the predetermined region often occurs during silver strike plating. More particularly, in silver plating on copper or copper alloy substrates, the adhesion between the silver coating and the substrates can be enhanced by carrying out silver strike plating in a bath having a low silver concentration and a high cyanide concentration prior to silver plating in a conventional silver plating bath. Since the strike plating bath has high penetrating power, silver coatings are often deposited on the unnecessary regions despite masking. One way for restricting such overcoating is to eliminate the silver strike plating, but without silver strike plating, the adhesion between the silver coating and the underlying substrates remains low.
In stripping silver coatings, it is desired that the underlying metal such as copper or copper alloys be not attacked or deteriorated in outer appearance. Another attempt is made for stripping unnecessary portions of a silver coating by exposing the overall silver coating to a stripping solution without masking the necessary portions. In this attempt, since the necessary portions of the silver coating are partially removed, it is desired that the necessary portions of the silver coating be attacked or deteriorated in outer appearance as scarcely as possible.
Prior art solutions for electrolytically stripping silver coatings include concentrated sulfuric acid having NaNO.sub.3 added thereto, solutions based on potassium or sodium cyanide, and solutions based on sodium thiosulfate. All these solutions suffer from problems that they etch the underlying metal such as copper and copper alloys, cause gloss variations or deterioration or satinizing on the necessary silver coating portions, and deteriorate the appearance of the underlying metal or the necessary silver coating portions. Use of dangerous concentrated sulfuric acid and toxic cyanides is also undesirable.